I think it would be stupid for Sony to buy any publisher, pretty much all of the western ones are poorly run and full of dead weight, many of them can hardly get games out of the door anymore. Sony biggest advantage right now is how productive and well oiled their whole gaming operation is, it would be a massive step back to get involved with studios that don't have it figured out.
I think they pretty much have the AAA games covered, what they need right now, specially after the PS+ restructure is to invest in smaller studios, helping them take that next step even if they don't necessarily buy them. Studios like Amber Labs, the guys behind Outer Wilds, that studio that made FTL/Into the Breach, Hello Games and so on.
Some quality indie games, with a premium look day one on PS+ make a lot more sense for the service than day one AAA games. They could even work with these quality small studios on PlayStation IPs that are not being used as a way to get them more visibility. These could be published under a different label like movie studios do.
They really need to stop the partnerships with unproven studios like IllFonic and Lucid Games, that is just not working at all.